Cross-linked ethylenic polymer foam structures and process for making

ABSTRACT

Disclosed is a cross-linked ethylenic polymer foam structure of an ethylenic polymer material of a cross-linked, substantially linear ethylenic polymer. The substantially linear ethylenic polymer having in an uncross-linked state has: a) a melt flow ratio, I 10  /I 2 , ≧5.63; b) a molecular weight distribution, M w  /M n , defined by the equation: M w  /M n  &lt;(I 10  /I 2 )-4.63; and c) a critical shear rate at onset of surface melt fracture of at least 50 percent greater than the critical shear rate at the onset of surface melt fracture of a linear ethylenic polymer having about the same I 2  and M w  /M n . Further disclosed is a process for making the above foam structure.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending application Ser. No.08/148,711, filed Nov. 8, 1993, now abandoned, which is a continuationof application Ser. No. 08/054,329, filed Apr. 28, 1993, now U.S. Pat.No. 5,288,762, issued Feb. 22, 1994.

Related applications include U.S. Ser. Nos. 07/939,281, filed Sep. 2,1992 and 07/776,130, filed Oct. 15, 1991.

BACKGROUND OF THE INVENTION

This invention relates to a composition of and a process formanufacturing a cross-linked ethylenic polymer foam structure havingenhanced processability and physical properties.

Commercially-available, cross-linked ethylenic polymer foam structures(XLPE) are typically made with conventional low density polyethylene(LDPE) resins made by high pressure processes. LDPE resins have foundfavor because of their relatively low melt temperatures and desirableprocessability.

It would be desirable to have an XLPE foam structure with enhancedstrength and resiliency, yet retain the low foam density and desirableresin processability afforded by the use of LDPE resins. One means ofenhancing strength and resiliency is to employ a conventional linear lowdensity polyethylene (LLDPE), a copolymer of ethylene and a C₄₋₁₀alpha-olefin. Unfortunately, LLDPE resin is difficult to process and toa XLPE foam, and results in a relatively high density foam structurewith poor quality. It would be desirable to have an XLPE foam structuresimilar in strength and resiliency to an XLPE foam structure of LLDPEwhile comprising a resin having in an uncross-linked stateprocessability similar to an LDPE resin.

SUMMARY OF THE INVENTION

According to the present invention, there is a cross-linked ethylenicpolymer foam structure of an ethylenic polymer material of across-linked, substantially linear ethylenic polymer. The substantiallylinear ethylenic polymer has in an uncross-linked state a: a) a meltflow ratio, I₁₀ /I₂, ≧5.63; b) a molecular weight distribution, M_(w)/M_(n), defined by the equation: M_(w) /M_(n) ≦(I₁₀ /I₂)-4.63; and c) acritical shear rate at onset of surface melt fracture of at least 50percent greater than the critical shear rate at the onset of surfacemelt fracture of a linear ethylenic polymer having about the same I₂ andM_(w) /M_(n). The foam structure is strong, resilient, and of lowdensity. The structure has high strength and resiliency similar to thatof a foam structure comprised of cross-linked LLDPE, and has low densityand quality similar to that of a foam structure of cross-linked LDPE.

Further according to the present invention, there is a process formaking a cross-linked ethylenic polymer foam structure. First, afoamable melt polymer material is formed by blending and heating adecomposable chemical blowing agent and an ethylenic polymer materialcomprising a substantially ethylenic polymer having: a) a melt flowratio, I₁₀ /I₂, ≧5.63; b) a molecular weight distribution, M_(w) /M_(n),defined by the equation: M_(w) /M_(n) ≦(I₁₀ /I₂)-4.63; and c) a criticalshear rate at onset of surface melt fracture of at least 50 percentgreater than the critical shear rate at the onset of surface meltfracture of a linear ethylenic polymer having about the same I₂ andM_(w) /M_(n). Second, cross-linking is induced in the foamable meltpolymer material. Third, the foamable melt polymer material is expandedby exposing it to an elevated temperature to form the foam structure.The substantially linear ethylenic polymer has better processability andfoamability than LLDPE and similar to that of LDPE.

DETAILED DESCRIPTION

The present cross-linked foam structure offers a number of advantagesover cross-linked foam structures of the prior art. The presentstructure has desirable high strength and resiliency similar to that ofa foam structure of cross-linked LLDPE, and has a desirable low densitysimilar to that of a foam structure of cross-linked LDPE. The presentstructure has greater strength and resiliency than a cross-linked LDPEfoam structure, can be made softer as well. The present structure canhave softness similar to foam structures of a soft ethylene copolymer,e.g. ethylene/vinyl acetate (EVA) copolymer or a blend ofpolyvinylchloride and a nitrile rubber, but has greater strength andresiliency. In addition, the present structure, being more thermallystable than an EVA foam structure, does not liberate unpleasant smellduring foam expansion, fabrication, and usage. Soft, strong cross-linkedfoam structures are useful in sporting good, medical device, andcushioning applications.

In the following paragraphs, the chemical and physical characteristicsof the substantially linear ethylenic polymer in a substantiallynon-crosslinked or uncross-linked state and methods for making same aredescribed. The substantially linear ethylenic polymer is cross-linkedand expanded or foamed to form the present cross-linked foam structure.The desirable characteristics of the non-crosslinked or uncross-linkedpolymer aid in extrusion and processing of the polymer prior to andduring cross-linking and foaming.

Elastic, substantially linear ethylenic polymers have unusualproperties, including an unusual combination of properties, which leadsto enhanced processability of the novel polymers. The substantiallylinear ethylenic polymers have processability similar to highly branchedlow density polyethylene, but the strength and toughness similar tolinear low density polyethylene.

The substantially linear ethylenic polymers preferably have a criticalshear rate at onset of surface melt fracture of at least 50 percentgreater than the critical shear rate at the onset of surface meltfracture of a linear ethylenic polymer having about the same I₂ andM_(w) /M_(n).

The elastic substantially linear ethylenic polymers also have aprocessing index (PI) less than or equal to about 70 percent of the PIof a comparative linear ethylenic polymer at about the same I₂ and M_(w)/M_(n).

The elastic substantially linear ethylenic polymers also preferably havea melt flow ratio, I₁₀ /I₂, ≧5.63, and preferably a molecular weightdistribution, M_(w) /M_(n), defined by the equation: M_(w) /M_(n) ≦(I₁₀/I₂)-4.63.

Elastic substantially linear ethylenic polymers comprising ethylenehomopolymers or a copolymer of ethylene with at least one C₃ -C₂₀α-olefinic (alpha-olefinic) comonomer are especially preferred.

Other properties of the substantially linear polymers include: a) adensity preferably from about 0.85 grams/cubic centimeter (g/cm³) toabout 0.97 g/cm³ ; and b) a melt index, I₂, preferably from about 0.01grams/10 minutes to about 1000 gram/10 minutes.

Preferably the melt flow ratio, I₁₀ /I₂, is from about 7 to about 20.

The molecular weight distribution (M_(w) /M_(n)) is preferably about 3.5or less, more preferably from about 1.5 to about 2.5, and mostpreferably from about 1.7 to about 2.3.

Throughout this disclosure, "melt index" or "I₂ " is measured inaccordance with ASTM D-1238 (190° C./2.16 kg); "I₁₀ " is measured inaccordance with ASTM D-1238 (190° C./10 kg).

The melt tension of these new polymers is also surprisingly good, e.g.,as high as about 2 grams or more, especially for polymers which have avery narrow molecular weight distribution (i.e., M_(w) /M_(n) from about1.5 to about 2.5).

A preferred substantially linear ethylenic polymer useful in the presentfoam structure has the following properties: a melt flow ratio index I₂(measured per ASTM method D-1238 condition E) of 0.5 to 20 dg/min,preferably, from 2.5 to 10 dg/min; a melt index ratio, I₁₀ /I₂ (per ASTMD1238) greater than 7.5, more preferably greater than 9; a single DSC(differential scanning calorimetry) melting peak.

The substantially linear polymers can be homopolymers of ethylene(polyethylene) or they can be copolymers of ethylene with at least oneC₃ -C₂₀ α-olefinic and/or C₂ -C₂₀ acetylenically compounds and/or C₄-C₁₈ diolefinics and/or other unsaturated comonomers. Useful comonomersinclude ethylenically unsaturated monomers, conjugated or nonconjugateddienes, polyenes, etc. Preferred monomers include the C₃ -C₁₀α-olefinics especially 1-propene, isobutylene, 1-butene, 1-hexene,4-methyl-1-pentene, and 1-octene. Other useful comonomers includestyrene, halo- or alkyl substituted styrenes, tetrafluoroethylene,vinylbenzocyclobutane, 1,4-hexadiene, and naphtenics (e.g.,cyclo-pentene, cyclo-hexene and cyclo-octene).

The term "substantially linear polymers" means that the polymer backboneis substituted with about 0.01 long chain branches/1000 carbons to about3 long chain branches/1000 carbons, more preferably from about 0.01 longchain branches/1000 carbons to about 1 long chain branches/1000 carbons,and especially from about 0.05 long chain branches/1000 carbons to about1 long chain branches/1000 carbons.

The term "linear ethylenic polymers" means that the ethylenic polymerdoes not have long chain branching. That is, the linear ethylenicpolymer has an absence of long chain branching, as for example theconventional linear low density polyethylene polymers or linear highdensity polyethylene polymers made using Ziegler polymerizationprocesses (for example, U.S. Pat. No. 4,076,698 or 3,645,992, both ofwhich are incorporated herein by reference). The term "linear ethylenicpolymers" does not refer to branched ethylenic polymers such as branchedpolyethylene, ethylene/vinyl acetate copolymers, or ethylene/estercopolymers which are made by high pressure processes and known to thoseskilled in the art to have numerous long chain branches.

Long chain branching is defined herein as a chain length of at leastabout 6 carbons, above which the length cannot be distinguished using ¹³C nuclear magnetic resonance (NMR) spectroscopy. The long chain branchcan be as long as about the same length as the length of the polymerback-bone.

Long chain branching is determined by using ¹³ C NMR spectroscopy and isquantified using the method of Randall (Rev. Macromol. Chem. Phys., C29(2&3), p. 285-297), the disclosure of which is incorporated herein byreference.

Excellent teachings regarding substantially linear ethylenic polymersand processes for making are seen in U.S. Ser. Nos. 07/939,281, filedSep. 9, 1992 and 07/776,130, filed Oct. 15, 1992, both of which areincorporated herein by reference.

"Melt tension" is measured by a specially designed pulley transducer inconjunction with the melt indexer. Melt tension is the load that theextrudate or filament exerts while passing over the pulley at thestandard speed of 30 rpm. The melt tension measurement is similar to the"Melt Tension Tester" made by Toyoseiki and is described by John Dealyin Rheometers for Molten Plastics, published by Van Nostrand ReinholdCo. (1982) on page 250-251.

The SCBDI (Short Chain Branch Distribution Index) or CDBI (CompositionDistribution Branch Index) is defined as the weight percent of thepolymer molecules having a comonomer content within 50 percent of themedian total molar comonomer content. The CDBI of a polymer is readilycalculated from data obtained from techniques known in the art, such as,for example, temperature rising elution fractionation (abbreviatedherein as "TREF") as described, for example, in Wild et al, Journal ofPolymer Science, Poly, Phys. Ed., Vol. 20, p. 441 (1982), or in U.S.Pat. No. 4,798,081, both disclosures of which are incorporated herein byreference. The SCBDI or CDBI for the substantially linear ethylenicpolymers of the present invention is preferably greater than about 30percent, especially greater than about 50 percent. Thus, thesubstantially linear polymers are homogeneously branched.

A unique characteristic of the present polymers is a highly unexpectedflow property where the I₁₀ /I₂ value is essentially independent ofpolydispersity index (i.e. M_(w) /M_(n)). This is contrasted withconventional polyethylene resins having rheological properties such thatas the polydispersity index increases, the I₁₀ /I₂ value also increases.For the substantially linear ethylenic polymers described herein, theI₁₀ /I₂ value indicates the amount of long chain branching (i.e., thehigher the I₁₀ /I₂ value, the more long chain branching).

The density of the ethylene or ethylene/α-olefinic substantially linearethylenic polymers is measured in accordance with ASTM D-792 and isgenerally from about 0.85 g/cm³ to about 0.97 g/cm³ and preferably fromabout 0.87 g/cm³ to about 0.95 g/cm³.

The molecular weight of the ethylene or ethylene/α-olefinicsubstantially linear ethylenic polymers in the present invention isconveniently indicated using a melt index measurement according to ASTMD-1238, Condition 190° C./2.16 kg (formally known as "Condition (E)" andalso known as I₂). Melt index is inversely related to the molecularweight of the polymer. Thus, the higher the molecular weight, the lowerthe melt index, although the relationship is not linear. The melt indexfor the ethylene or ethylene/α-olefinic substantially linear ethylenicpolymers used herein is generally from about 0.01 grams/10 minutes (g/10min) to about 1000 g/10 min, preferably from about 0.05 g/10 min toabout 100 g/10 min, and especially from about 0.1 g/10 min to about 20g/10 min.

Additives such as antioxidants (e.g., hindered phenolics (e.g., Irganox®1010), phosphites (e.g., Irgafos® 168)), pigments, and the like can alsobe included in the polyethylene compositions, to the extent that they donot interfere with the enhanced properties discovered by applicants.

Ethylenic polymer materials of blends of the substantially linearethylenic polymer and a suitable different ethylenic polymer or othernatural or synthetic polymers are possible. Suitable different ethylenicpolymers include low (LDPE) (for example, high pressure, free-radicalpolymerization techniques), medium (MDPE), and high densitypolyethylenes (HDPE) (for example, those made using Ziegler catalysts asin U.S. Pat. No. 4,076,698), ethylene/ester copolymers, ethylene/vinylacetate copolymers, copolymers of ethylene and ethylenically unsaturatedcarboxylic acid, homo and copolymers of alpha-ethylenics, etc. Othersuitable polymers include polystyrene (including high impactpolystyrene, styrene-butadiene block copolymers, polyisoprene, and otherrubbers. Blends comprising a major proportion of the resin having thehigher melting point are preferred. Regardless of composition, theethylenic polymer material preferably comprises greater than 50 and morepreferably greater than 70 weight percent of ethylenic monomeric units.The ethylenic polymer material may be comprised completely or entirelyof ethylenic monomeric units. Preferred blends are those with thesubstantially linear ethylenic polymer and other conventional ethylenicpolymers such as LDPE, HDPE, ethylene/acrylic acid copolymer (EAA), andLLDPE.

The improved melt elasticity and processibility of the substantiallylinear polymers according to the present invention result, it isbelieved, from their method of production. The polymers may be producedvia a continuous (as opposed to a batch) controlled polymerizationprocess using at least one reactor, but can also be produced usingmultiple reactors (e.g., using a multiple reactor configuration asdescribed in U.S. Pat. No. 3,914,342) at a polymerization temperatureand pressure sufficient to produce the polymers having the desiredproperties. According to one embodiment of the present process, thepolymers are produced in a continuous process, as opposed to a batchprocess. Preferably, the polymerization temperature is from about 20° C.to about 250° C., using constrained geometry catalyst technology. If anarrow molecular weight distribution polymer (M_(w) /M_(n) of from about1.5 to about 2.5) having a higher I₁₀ /I₂ ratio (e.g. I₁₀ /I₂ of about 7or more, preferably about 8 or more, more preferably about 9 or more) isdesired, the ethylene concentration in the reactor is preferably notmore than about 8 percent by weight of the reactor contents, especiallynot more than about 4 percent by weight of the reactor contents.Preferably, the polymerization is performed in a solution polymerizationprocess. Generally, manipulation of I₁₀ /I₂ while holding M_(w) /M_(n)relatively low for producing the novel polymers described herein is afunction of reactor temperature and/or ethylene concentration. Reducedethylene concentration and higher temperature generally produces higherI₁₀ /I₂.

The molecular weight distribution (M_(w) /M_(n)) of the individualpolymer samples is analyzed by gel permeation chromatography (GPC) on aWaters 150° C. high temperature chromatographic unit equipped with threemixed porosity columns (Polymer Laboratories 10³, 10⁴, 10⁵, and 10⁶),operating at a system temperature of 140° C. The solvent is1,2,4-trichlorobenzene, from which 0.3 percent by weight solutions ofthe samples are prepared for injection. The flow rate is 1.0milliliters/minute and the injection size is 200 microliters.

The molecular weight determination is deduced by using narrow molecularweight distribution polystyrene standards (from Polymer Laboratories) inconjunction with their elution volumes. The equivalent polyethylenemolecular weights are determined by using appropriate Mark-Houwinkcoefficients for polyethylene and polystyrene (as described by Williamsand Word in Journal of Polymer Science, Polymer Letters, Vol. 6, (621)1968, incorporated herein by reference) to derive the followingequation: M_(polyethylene) =a* (M_(polystyrene))^(b). In this equation,a=0.4316 and b=1.0. Weight average molecular weight, M_(w), iscalculated in the usual manner according to the following formula: M_(w)=Σw_(i) *M_(i), where w_(i) and M_(i) are the weight fraction andmolecular weight, respectively, of the i^(th) fraction eluting from theGPC column.

The rheological processing index (PI) is measured by a gas extrusionrheometer (GER). The GER is described by M. Shida, R. N. Shroff and L.V. Cancio in Polym. Eng. Sci., Vol. 17, no. 11, p. 770 (1977), and inRheometers for Molten Plastics by John Dealy published by Van NostrandReinhold Co. (1982) on page 97-99, the disclosures of both of which areincorporated herein by reference. The processing index is measured at atemperature of 190° C., at nitrogen pressure of 2500 psig using 0.0296inch diameter, 20:1 L/D die having an entrance angle of 180 degrees. TheGER processing index is calculated in millipoise units from thefollowing equation:

PI=2.15×10⁶ dynes/cm² /(1000×shear rate), wherein:

2.15×10⁶ dynes/cm² is the shear stress at 2500 psi, and the shear rateis the shear rate at the wall as represented by the following equation:

32 Q'/(60 sec/min)(0.745)(Diameter×2.54 cm/in)³, wherein:

Q' is the extrusion rate (gms/min),

0.745 is the melt density of polyethylene (gm/cm³), and

Diameter is the orifice diameter of the capillary (inches).

The PI is the apparent viscosity of a material measured at apparentshear stress of 2.15×10⁶ dyne/cm².

For the substantially linear ethylenic polymers disclosed herein, the PIis less than or equal to 70 percent of that of a comparative linearethylenic polymer at about the same I₂ and M_(w) /M_(n).

Suitable constrained geometry catalysts for use herein preferablyinclude constrained geometry catalysts as disclosed in U.S. Serial Nos.:545,403, filed Jul. 3, 1990; 758,654, filed Sep. 12, 1991; 758,660,filed Sep. 12, 1991; and 720,041, filed Jun. 24, 1991, the teachings ofall of which are incorporated herein by reference. Themonocyclopentadienyl transition metal ethylenic polymerization catalyststaught in U.S. Pat. No. 5,026,798, the teachings of which areincorporated herein by reference, are also suitable for use in preparingthe polymers of the present invention.

Constrained-geometry catalysts useful in making the substantially linearethylenic polymer comprise a metal complex and a cocatalyst.

Metal complexes correspond to the Formula (I): ##STR1## wherein: M isselected from the group consisting of groups 3-10 or the Lanthanideseries of the Periodic Table of the Elements;

Cp* is selected from the group consisting of indenyl, tetrahydroindenyl,fluorenyl, octahydrofluorenyl, tetrahydrofluorenyl, cyclopentadienyl,and R-substituted cyclopentadienyl bound in an η⁵ bonding mode to M;

X is, independently each occurrence, an anionic ligand group selectedfrom the group consisting of hydride, halide, alkyl of up to 30 carbonatoms, alkoxy having up to a total of 30 carbon and oxygen atoms,cyanide, azide, acetylacetonate, aryl of up to 30 carbon atoms, aryloxyhaving up to a total of 30 carbon and oxygen atoms, norbornyl, andbenzyl;

n is 0, 1, 2, 3, or 4 and is 2 less than the valence of M;

Y is NR", PR", 0, or S;

Z is CR₂ *, CR₂ * CR₂ *, SIR₂ *, SiR₂ * SiR₂ *;

R is selected from the group consisting of alkyl having 1 to 20 carbonatoms, cyano, norbornyl, benzyl, aryl of up to 20 carbon atoms, (R')₃Si, and (R')₃ Ge;

R' is selected from the group consisting of C₁ -C₂₀ alkyl and aryl of upto 20 carbon atoms;

R" is selected from the group consisting of C₁ -C₂₀ alkyl, aryl of up to20 carbon atoms, benzyl, haloaryl having up to a total of 20 carbon andhalogen atoms, 2-methoxyphenyl, 4-methoxyphenyl, and norbornyl; and

R* is selected from the group consisting of hydrogen, C₁ -C₂₀ alkyl,haloalkyl having up to a total of 20 carbon and halogen atoms, aryl ofup to 20 carbon atoms, and haloaryl of up to a total of 20 carbon andhalogen atoms.

Preferred metal complexes are those of Formula (I) wherein:

M is titanium, zirconium, or hafnium;

X is, independently each occurrence, selected from the group consistingof halide, alkyl of up to 30 carbon atoms, aryl of up to 30 carbonatoms, and benzyl;

n is 1 or 2;

Y is NR";

Z is CR₂ *CR₂ * or SIR₂ *;

R is selected from the group consisting of alkyl having 1 to 20 carbonatoms, benzyl, aryl of up to 20 carbon atoms, and (R')₃ Si;

R' is selected from the group consisting of C₁ -C₂₀ alkyl and aryl of upto 20 carbon atoms;

R" is selected from the group consisting of C₁ -C₂₀ alkyl, aryl of up to20 carbon atoms, and benzyl; and

R* is selected from the group consisting of hydrogen, C₁ -C₂₀ alkyl, andaryl of up to 20 carbon atoms.

It should be noted that the complex may exist as a dimer or higheroligomer. Further preferably, at least one of R, Z, or R" is an electrondonating moiety. Thus, highly preferably Y is a nitrogen or phosphoruscontaining group corresponding to the formula --NR"-- or --PR"-- whereinR" is C₁ -C₂₀ alkyl, i.e., an alkyl amido or alkyl phosphido group. Mosthighly preferred complex compounds are amidosilane- or amidoalkanediyl-compounds corresponding to Formula (II): ##STR2## wherein: M istitanium, zirconium or hafnium, bound in an η⁵ bonding mode to thecyclopentadienyl group;

R' each occurrence is independently selected from the group consistingof hydrogen, silyl, alkyl, aryl and combinations thereof, said R' havingup to 10 carbon or silicon atoms;

E is silicon or carbon;

X independently each occurrence is hydride, halo, alkyl, aryl, aryloxyor alkoxy, said X having up to 10 carbons;

m is 1 or 2; and

n is 1 or 2.

Examples of the above most highly preferred metal coordination compoundsinclude compounds wherein the R' on the amido group is methyl, ethyl,propyl, butyl, pentyl, hexyl, (including isomers), norbornyl, benzyl orphenyl; the cyclopentadienyl group is cyclopentadienyl, indenyl,tetrahydroindenyl fluorenyl, or octahydrofluorenyl; R' on the foregoingcyclopentadienyl groups each occurrence is hydrogen, methyl, ethyl,propyl, butyl, pentyl, hexyl, (including isomers), norbornyl, benzyl, orphenyl; and X is chloro, bromo, iodo, methyl, ethyl, propyl, butyl,pentyl, hexyl, (including isomers), norbornyl, benzyl, or phenyl.

Specific compounds include: (tert-butylamido)(tetramethyl-η⁵-cyclopentadienyl)-1,2-ethanediylzirconium dichloride,(tert-butylamido)(tetramethyl-η⁵-cyclopentadienyl)-1,2-ethanediyltitanium dichloride,(methylamido)(tetramethyl-η⁵ -cyclopentadienyl)-1,2-ethanediylzirconiumdichloride, (methylamido)(tetramethyl-η⁵-cyclopentadienyl)-1,2-ethanediyltitanium dichloride,(ethylamido)(tetramethyl-η⁵ -cyclopentadienyl)-methylenetitaniumdichloro, (tertbutylamido)dibenzyl(tetramethyl-η⁵ -cyclopentadienyl)silanezirconium dibenzyl, (benzylamido)dimethyl- (tetramethyl-η⁵-cyclopentadienyl)silanetitanium dichloride,(phenylphosphido)dimethyl(tetramethyl-η⁵-cyclopentadienyl)silanezirconium dibenzyl,(tertbutylamido)dimethyl(tetramethyl-η⁵ -cyclopentadienyl)silanetitaniumdimethyl, and the like.

The complexes may be prepared by contacting a derivative of a metal, M,and a group I metal derivative or Grignard derivative of thecyclopentadienyl compound in a solvent and separating the saltbyproduct. Suitable solvents for use in preparing the metal complexesare aliphatic or aromatic liquids such as cyclohexane,methylcyclohexane, pentane, hexane, heptane, tetrahydrofuran, diethylether, benzene, toluene, xylene, ethylbenzene, etc., or mixturesthereof.

In a preferred embodiment, the metal compound is MX_(n+1), i.e., M is ina lower oxidation state than in the corresponding compound, MX_(n+2) andthe oxidation state of M in the desired final complex. A noninterferingoxidizing agent may thereafter be employed to raise the oxidation stateof the metal. The oxidation is accomplished merely by contacting thereactants utilizing solvents and reaction conditions use in thepreparation of the complex itself. By the term "noninterfering oxidizingagent" is meant a compound having an oxidation potential sufficient toraise the metal oxidation state without interfering with the desiredcomplex formation or subsequent polymerization processes. A particularlysuitable noninterfering oxidizing agent is AgCl or an organic halidesuch as methylene chloride. The foregoing techniques are disclosed inU.S. Ser. Nos. 545,403, filed Jul. 3, 1990 and 702,475, filed May 20,1991, the teachings of both of which are incorporated herein byreference.

Additionally the complexes may be prepared according to the teachings ofthe copending U.S. Ser. No. 778,433 entitled: "Preparation of MetalCoordination Complex (I)", filed in the names of Peter Nickias and DavidWilson, on Oct. 15, 1991 and the copending U.S. Ser. No. 778,432entitled: "Preparation of Metal Coordination Complex (II)", filed in thenames of Peter Nickias and David Devore, on Oct. 15, 1991, the teachingsof which are incorporated herein by reference thereto.

Suitable cocatalysts for use herein include polymeric or oligomericaluminoxanes, especially methyl aluminoxane, as well as inert,compatible, noncoordinating, ion forming compounds. So-called modifiedmethyl aluminoxane (MMAO) is also suitable for use as a cocatalyst. Onetechnique for preparing such modified aluminoxane is disclosed in U.S.Pat. No. 5,041,584, the teachings of which are incorporated herein byreference. Aluminoxanes can also be made as disclosed in U.S. Pat. Nos.5,542,199, 4,544,762, 5,015,749, and 5,041,585, the entire specificationof each of which is incorporated herein by reference. Preferredcocatalysts are inert, noncoordinating, boron compounds.

Ionic active catalyst species which can be used to polymerize thepolymers described herein correspond to Formula (III): ##STR3## wherein:M, Cp*, X, Y, and Z are as defined for Formula (I) above;

n is 1, 2, 3, or 4 and is 2 less than the oxidation state of M; and

A- is a noncoordinating, compatible anion.

As used herein, the recitation "noncoordinating, compatible anion" meansan anion which either does not coordinate to the monocyclopentadienyl orsubstituted monocyclopentadienyl group containing cation or which isonly weakly coordinated to said cation thereby remaining sufficientlylabile to be displaced by a neutral Lewis base. A noncoordinating,compatible anion specifically refers to a compatible anion which whenfunctioning as a charge balancing anion in the catalyst system of thisinvention does not transfer an anionic substituent or fragment thereofto said cation thereby forming a neutral four coordinate metallocene anda neutral metal byproduct. "Compatible anions" are anions which are notdegraded to neutrality when the initially formed complex decomposes andare noninterfering with desired subsequent polymerization or other usesof the complex.

One method of making the ionic catalyst species which can be utilized tomake the polymers of the present invention involve combining: a) atleast one first component which is a mono(cyclopentadienyl) derivativeof a metal of Group 3-10 or the Lanthanide Series of the Periodic Tableof the Elements containing at least one substituent which will combinewith the cation of a second component (described hereinafter) whichfirst component is capable of forming a cation formally having acoordination number that is one less than its valence; and b) at leastone second component which is a salt of a Bronsted acid and anoncoordinating, compatible anion.

More particularly, the non-coordinating, compatible anion of theBronsted acid salt may comprise a single coordination complex comprisinga charge-bearing metal or metalloid core, which anion is both bulky andnon-nucleophilic. The recitation "metalloid", as used herein, includesnon-metals such as boron, phosphorus and the like which exhibitsemi-metallic characteristics.

Illustrative, but not limiting examples of monocyclopentadienyl metalcomponents (first components) which may be used in the preparation ofcationic complexes are derivatives of titanium, zirconium, vanadium,hafnium, chromium, lanthanum, etc. Preferred components are titanium orzirconium compounds. Examples of suitable monocyclopentadienyl metalcompounds are hydrocarbyl-substituted monocyclopentadienyl metalcompounds such as ( tert-butylamido)(tetramethyl-η⁵-cyclopentadienyl)-1,2-ethanediylzirconium dimethyl, (tert-butylamido)(tetramethyl-η⁵ -cyclopentadienyl)-1,2-ethanediyltitanium dimethyl,(methylamido)(tetramethyl-η⁵ -cyclopentadienyl)-1,2-ethanediylzirconiumdibenzyl, (methylamido)(tetramethyl-η⁵-cyclopentadienyl)-1,2-ethanediyltitanium dimethyl,(ethylamido)(tetramethyl-η⁵ -cyclopentadienyl)methylenetitaniumdimethyl, (tertbutylamido)dibenzyl(tetramethyl-η⁵ -cyclopentadienyl)silanezirconium dibenzyl, (benzylamido)dimethyl- (tetramethyl-η⁵-cyclopentadienyl)silanetitanium diphenyl,(phenylphosphido)dimethyl(tetramethyl-η⁵-cyclopentadienyl)silanezirconium dibenzyl, and the like.

Such components are readily prepared by combining the correspondingmetal chloride with a dilithium salt of the substituted cyclopentadienylgroup such as a cyclopentadienyl-alkanediyl, cyclopentadienyl-silaneamide, or cyclopentadienyl-phosphide compound. The reaction is conductedin an inert liquid such as tetrahydrofuran, C₅₋₁₀ alkanes, toluene, etc.utilizing conventional synthetic procedures. Additionally, the firstcomponents may be prepared by reaction of a group II derivative of thecyclopentadienyl compound in a solvent and separating the saltby-product. Magnesium derivatives of the cyclopentadienyl compounds arepreferred. The reaction may be conducted in an inert solvent such ascyclohexane, pentane, tetrahydrofuran, diethyl ether, benzene, toluene,or mixtures of the like. The resulting metal cyclopentadienyl halidecomplexes may be alkylated using a variety of techniques. Generally, themetal cyclopentadienyl alkyl or aryl complexes may be prepared byalkylation of the metal cyclopentadienyl halide complexes with alkyl oraryl derivatives of group I or group II metals. Preferred alkylatingagents are alkyl lithium and Grignard derivatives using conventionalsynthetic techniques. The reaction may be conducted in an inert solventsuch as cyclohexane, pentane, tetrahydrofuran, diethyl ether, benzene,toluene, or mixtures of the like. A preferred solvent is a mixture oftoluene and tetrahydrofuran.

Compounds useful as a second component in the preparation of the ioniccatalysts useful in this invention will comprise a cation, which is aBronsted acid capable of donating a proton, and a compatiblenoncoordinating anion. Preferred anions are those containing a singlecoordination complex comprising a charge-bearing metal or metalloid corewhich anion is relatively large (bulky), capable of stabilizing theactive catalyst species (the Group 3-10 or Lanthanide Series cation)which is formed when the two components are combined and sufficientlylabile to be displaced by ethylenicic, diethylenicic and acetylenicallyunsaturated substrates or other neutral Lewis bases such as ethers,nitriles and the like. Suitable metals, then, include, but are notlimited to, aluminum, gold, platinum and the like. Suitable metalloidsinclude, but are not limited to, boron, phosphorus, silicon and thelike. Compounds containing anions which comprise coordination complexescontaining a single metal or metalloid atom are, of course, well knownand many, particularly such compounds containing a single boron atom inthe anion portion, are available commercially. In light of this, saltscontaining anions comprising a coordination complex containing a singleboron atom are preferred.

Highly preferably, the second component useful in the preparation of thecatalysts of this invention may be represented by the following generalformula:

(L-H)⁺ [A]⁻

wherein:

L is a neutral Lewis base;

(L-H)⁺ is a Bronsted acid; and

[A]⁻ is a compatible, noncoordinating anion.

More preferably [A]⁻ corresponds to the formula:

[M'Q_(q) ]⁻

wherein:

M' is a metal or metalloid selected from Groups 5-15 of the PeriodicTable of the Elements;

Q independently each occurrence is selected from the Group consisting ofhydride, dialkylamido, halide, alkoxide, aryloxide, hydrocarbyl, andsubstituted-hydrocarbyl radicals of up to 20 carbons with the provisothat in not more than one occurrence is Q halide; and

q is one more than the valence of M'.

Second components comprising boron which are particularly useful in thepreparation of catalysts of this invention may be represented by thefollowing general formula:

[L-H]⁺ [BQ₄ ]⁻

wherein:

L is a neutral Lewis base;

[L-H]⁺ is a Bronsted acid;

B is boron in a valence state of 3; and

Q is as previously defined.

Illustrative, but not limiting, examples of boron compounds which may beused as a second component in the preparation of the improved catalystsof this invention are trialkyl-substituted ammonium salts such astriethylammonium tetraphenylborate, tripropylammonium tetraphenylborate,tris(n-butyl)ammonium tetraphenylborate, trimethylammoniumtetrakis(p-tolyl)borate, tributylammoniumtetrakis(pentafluorophenyl)borate, tripropylammoniumtetrakis(2,4-dimethylphenyl)borate, tributylammoniumtetrakis(3,5-dimethyphenyl)borate, triethylammoniumtetrakis(3,5-di-trifluoromethylphenyl)borate and the like. Also suitableare N,N-dialkyl anilinium salts such asN,N-dimethyl-aniliniumtetraphenylborate, N,N-diethylaniliniumtetraphenylborate, N,N-dimethyl-2,4,6-trimethylaniliniumtetraphenylborate and the like; dialkylammonium salts such asdi-(i-propyl)ammonium tetrakis(pentafluorophenyl)borate,dicyclohexylammonium tetraphenylborate and the like; and triarylphosphonium salts such as triphenylphosphonium tetraphenylborate,tri(ethylphenyl)phosphonium tetrakispentafluorophenylborate,tri(dimethylphenyl)phosphonium tetraphenylborate and the like.

Preferred ionic catalysts formed in this manner are those having alimiting charge separated structure corresponding to Formula (IV):##STR4## wherein: M is a titanium, zirconium, or hafnium;

Cp*, X, and Z are as defined for Formula (I);

n is 1, 2, 3, or 4 and is 2 less than the oxidation state of M; and

XA*- is ⁻ XB(C₆ F₅)₃.

This class of cationic complexes may be conveniently prepared bycontacting a metal compound corresponding to Formula (V): ##STR5##wherein: Cp*, M, and n are as previously defined for Formula (IV), withtris(pentafluorophenyl)borane cocatalyst under conditions to causeabstraction of X and formation of the anion ⁻ XB(C₆ F₅)₃.

Preferably X in the foregoing ionic catalyst is C₁ -C₁₀ hydrocarbyl,most preferably methyl.

Formula (V) is referred to as the limiting, charge separated structure.However, it is to be understood that, particularly in solid form, thecatalyst may not be fully charge separated. That is, the X group mayretain a partial covalent bond to the metal atom, M. Thus, the catalystsmay be alternately depicted as possessing Formula (VI): ##STR6##

The catalysts are preferably prepared by contacting the derivative of aGroup 4 or Lanthanide metal with the tris(pentafluorophenyl)borane in aninert diluent such as an organic liquid. Tris(pentafluorophenyl)boraneis a commonly available Lewis acid that may be readily preparedaccording to known techniques. The compound is disclosed in Marks, etal. J. Am. Chem. Soc. 1991, 113, 3623-3625 for use in alkyl abstractionof zirconocenes.

All reference to the Periodic Table of the Elements herein shall referto the Periodic Table of the Elements, published and copyrighted by CRCPress, Inc., 1989. Also, any reference to a Group or Groups shall be tothe Group or Groups as reflected in this Periodic Table of the Elementsusing the IUPAC system for numbering groups.

Other compounds which are useful in the catalyst compositions of thisinvention, especially compounds containing other Group 4 or Lanthanidemetals, will, of course, be apparent to those skilled in the art.

The polymerization conditions for manufacturing the polymers of thepresent invention are generally those useful in the solutionpolymerization process, although the application of the presentinvention is not limited thereto. Slurry and gas phase polymerizationprocesses are also believed to be useful, provided the proper catalystsand polymerization conditions are employed.

Multiple reactor polymerization processes are also useful in the presentinvention, such as those disclosed in U.S. Pat. No. 3,914,342,incorporated herein by reference. The multiple reactors can be operatedin series or in parallel, with at least one constrained geometrycatalyst employed in at least one of the reactors.

In general, the continuous polymerization according to the presentinvention may be accomplished at conditions well known in the prior artfor Ziegler-Natta or Kaminsky-Sinn type polymerization reactions, thatis, temperatures from 0° to 250° C. and pressures from atmospheric to1000 atmospheres (100 MPa). Suspension, solution, slurry, gas phase orother process conditions may be employed if desired. A support may beemployed but preferably the catalysts are used in a homogeneous (i.e.,soluble manner. It will, of course, be appreciated that the activecatalyst system, form in situ if the catalyst and the cocatalystcomponents thereof are added directly to the polymerization process anda suitable solvent or diluent, including condensed monomer, is used insaid polymerization process. It is, however, preferred to form theactive catalyst in a separate step in a suitable solvent prior to addingthe same to the polymerization mixture.

The present foam structure may take any physical configuration known inthe art, such as sheet, plank, or bun stock. Other useful forms areexpandable or foamable particles, moldable foam particles, or beads, andarticles formed by expansion and/or coalescing and welding of thoseparticles.

Excellent teachings to processes for making ethylenic polymer foamstructures and processing them are seen in C. P. Park, "PolyolefinFoam", Chapter 9, Handbook of Polymer Foams and Technology, edited by D.Klempner and K. C. Frisch, Hanser Publishers, Munich, Vienna, New York,Barcelona (1991), which is incorporated herein by reference.

The present foam structure may be prepared by blending and heating anethylenic polymer material and a decomposable chemical blowing agent toform a foamable plasticized or melt polymer material, extruding thefoamable melt polymer material through a diet inducing cross-linking inthe melt polymer material, and exposing the melt polymer material to anelevated temperature to release the blowing agent to form the foamstructure. The polymer material and the chemical blowing agent may bemixed and melt blended by any means known in the art such as with anextruder, mixer, blender, or the like. The chemical blowing agent ispreferably dry-blended with the polymer material prior to heating thepolymer material to a melt form, but may also be added when the polymermaterial is in melt phase. Cross-linking may be induced by addition of across-linking agent or by radiation. Induction of cross-linking andexposure to an elevated temperature to effect foaming or expansion mayoccur simultaneously or sequentially. If a cross-linking agent is used,it is incorporated into the polymer material in the same manner as thechemical blowing agent. Further, if a cross-linking agent is used, thefoamable melt polymer material is heated or exposed to a temperature ofpreferably less than 150° C. to prevent decomposition of thecross-linking agent or the blowing agent and to prevent prematurecross-linking. If radiation cross-linking is used, the foamable meltpolymer material is heated or exposed to a temperature of preferablyless than 160° C. to prevent decomposition of the blowing agent. Thefoamable melt polymer material is extruded or conveyed through a die ofdesired shape to form a foamable structure. The foamable structure isthen cross-linked and expanded at an elevated or high temperature(typically, 150° C.-250° C.) such as in an oven to form a foamstructure. If radiation cross-linking is used, the foamable structure isirradiated to cross-link the polymer material, which is then expanded atthe elevated temperature as described above. The present structure canadvantageously be made in sheet or thin plank form according to theabove process using either cross-linking agents or radiation.

The present foam structure may also be made into a continuous plankstructure by an extrusion process utilizing a long-land die as describedin GB 2,145,961A, which is incorporated herein by reference. In thatprocess, the polymer, decomposable blowing agent and cross-linking agentare mixed in an extruder, heating the mixture to let the polymercross-link and the blowing agent to decompose in a long-land die; andshaping and conducting away from the foam structure through the die withthe foam structure and the die contact lubricated by a properlubrication material.

The present foam structure may also be formed into cross-linked foambeads suitable for molding into articles. To make the foam beads,discrete resin particles such as granulated resin pellets are: suspendedin a liquid medium in which they are substantially insoluble such aswater; impregnated with a cross-linking agent and a blowing agent at anelevated pressure and temperature in an autoclave or other pressurevessel; and rapidly discharged into the atmosphere or a region ofreduced pressure to expand to form the foam beads. A version is that thepolymer beads is impregnated with blowing agent, cooled down, dischargedfrom the vessel, and then expanded by heating or with steam. In aderivative of the above process, styrene monomer may be impregnated intothe suspended pellets along with the cross-linking agent to form a graftinterpolymer with the ethylenic polymer material. Blowing agent may beimpregnated into the resin pellets while in suspension or, alternately,in non-hydrous state. The expandable beads are then expanded by heatingwith steam and molded by the conventional molding method for theexpandable polystyrene foam beads.

The foam beads may then be molded by any means known in the art, such ascharging the foam beads to the mold, compressing the mold to compressthe beads, and heating the beads such as with steam to effect coalescingand welding of the beads to form the article. Optionally, the beads maybe pre-heated with air or other blowing agent prior to charging to themold. Excellent teachings of the above processes and molding methods areseen in C. P. Park, Supra, pp. 227-233, U.S. Pat. No. 3,886,100, U.S.Pat. No. 3,959,189, U.S. Pat. No. 4,168,353, and U.S. Pat. No.4,429,059, which are incorporated herein by reference. The foam beadscan also be prepared by preparing a mixture of polymer, cross-linkingagent, and decomposable mixtures in a suitable mixing device or extruderand form the mixture into pellets, and heat the pellets to cross-linkand expand.

There is another process for making cross-linked foam beads suitable formolding into articles. The ethylene polymer material is melted and mixedwith a physical blowing agent in a conventional foam extrusion apparatusto form an essentially continuous foam strand. The foam strand isgranulated or pelletized to form foam beads. The foam beads are thencross-linked by radiation. The cross-linked foam beads may then becoalesced and molded to form various articles as described above for theother foam bead process. Additional teachings to this process are seenin U.S. Pat. No. 3,616,365 and C. P. Park, Supra, pp. 224-228, which areincorporated herein by reference.

The present foam structure may be made in bun stock form by twodifferent processes. One process involves the use of a cross-linkingagent and the other uses radiation.

The present foam structure may be made in bun stock form by mixing theethylenic polymer material, a cross-linking agent, and a chemicalblowing agent to form a slab, heat the mixture in a mold so thecross-linking agent can cross-link the polymer material and the blowingagent can decompose, and expand by release of pressure in the mold.Optionally, the bun stock formed upon release of pressure may bere-heated to effect further expansion.

Cross-linked polymer sheet is made by eigher irradiating polymer sheetwith high energy beam or by heating a polymer sheet containing chemicalcross-linking agent. The cross-linked polymer sheet is cut into thedesired shapes and impregnated with N2 in a higher pressure at atemperature above the softening point of the polymer; releasing thepressure affects nucleation of bubbles and some expansion in the sheet.Reheat the sheet in a low pressure vessel under pressure above thesoftening point and release the pressure so that the foam can expand.

Blowing agents useful in making the present foam structure includedecomposable chemical blowing agents. Such chemical blowing agentsdecompose at elevated temperatures to form gases or vapors to blow thepolymer into foam form. The agent preferably takes a solid form so itmay be easily dry-blended with the polymer material. Chemical blowingagents include azodicarbonamide, azodiisobutyro-nitrile,benzenesulfonhydrazide, 4,4-oxybenzene sulfonylsemicarbazide, p-toluenesulfonyl semi-carbazide, barium azodicarboxylate,N,N'-dimethyl-N,N'-dinitrosoterephthalamide,N,N'-dinitrosopentamethylenetetramine, 4-4-oxybis(benzenesulfonylhydrazide), and trihydrazino triazine. Azodicarbonamideis preferred. Additional teachings to chemical blowing agents are seenin C. P. Park, Supra, pp. 205-208, and F. A. Shutov, "Polyolefin Foam",Handbook of Polymer Foams and Technology, pp. 382-402, D. Klemper and K.C. Frisch, Hanser Publishers, Munich, Vienna, New York, Barcelona(1991), which is incorporated herein by reference.

The chemical blowing agent is blended with the polymer material in anamount sufficient to evolve about 0.2 to about 5.0, preferably fromabout 0.5 to about 3.0, and most preferably from about 1.0 to 2.50 molesof gas or vapor per kilogram of polymer.

Cross-linking agents useful in making the present foam structure are theorganic peroxides. Useful organic peroxides cross-linking agents include1,1-di-t-butyl peroxy-3,3,5-trimethylcyclohexane, dicumyl peroxide,2,5-dimethyl-2,5-di(t-butyl peroxy) hexane, t-butyl-cumyl peroxide,α,α'-di(butyl peroxy)diisopropyl benzene, di-t-butyl peroxide, and2,5-dimethyl-2,5-di-(t-butyl peroxy) hexyne. Dicumyl peroxide is thepreferred agent. Additional teachings to organic peroxide cross-linkingagents are seen in C. P. Park, Supra, pp. 198-204, which is incorporatedherein by reference.

Cross-linking by radiation may be effected by any of the conventionaltypes. Useful radiation types include an electron beam or beta ray,gamma rays, x-rays, or neutron rays. Radiation is believed to effectcross-linking by generating polymer radicals, which may combine andcross-link. Additional teachings concerning radiation cross-linking areseen in C. P. Park, Supra, pp. 198-204, which is incorporated herein byreference.

In some processes for making the present structure, a physical blowingagent may be used. Physical blowing agents include organic and inorganicagents. Suitable inorganic blowing agents include carbon dioxide,nitrogen, argon, water, air, nitrogen, and helium. Organic blowingagents include aliphatic hydrocarbons having 1-9 carbon atoms, aliphaticalcohols having 1-3 carbon atoms, and fully and partially halogenatedaliphatic hydrocarbons having 1-4 carbon atoms. Aliphatic hydrocarbonsinclude methane, ethane, propane, n-butane, isobutane, n-pentane,isopentane, neopentane, and the like. Aliphatic alcohols includemethanol, ethanol, n-propanol, and isopropanol. Fully and partiallyhalogenated aliphatic hydrocarbons include fluorocarbons, chlorocarbons,and chlorofluorocarbons. Examples of fluorocarbons include methylfluoride, perfluoromethane, ethyl fluoride, 1,1-difluoroethane(HFC-152a), 1,1,1-trifluoroethane (HFC-143a), 1,1,1,2-tetrafluoro-ethane(HFC-134a), pentafluoroethane, difluoromethane, perfluoroethane,2,2-difluoropropane, 1,1,1-trifluoropropane, perfluoropropane,dichloropropane, difluoropropane, perfluorobutane, perfluorocyclobutane.Partially halogenated chlorocarbons and chlorofluorocarbons for use inthis invention include methyl chloride, methylene chloride, ethylchloride, 1,1,1-trichloroethane, 1,1-dichloro-1-fluoroethane(HCFC-141b), 1-chloro-1,1-difluoroethane (HCFC-142b),chlorodifluoromethane (HCFC-22), 1,1-dichloro-2,2,2-trifluoroethane(HCFC-123) and 1-chloro-1,2,2,2-tetrafluoroethane (HCFC-124). Fullyhalogenated chlorofluorocarbons include trichloromonofluoromethane(CFC-11), dichlorodifluoromethane (CFC-12), trichlorotrifluoroethane(CFC-113), 1,1,1-trifluoroethane, pentafluoroethane,dichlorotetrafluoroethane (CFC-114), chloroheptafluoropropane, anddichlorohexafluoropropane.

The amount of blowing agent incorporated into the polymer melt materialto make a foam-forming polymer gel is from about 0.2 to about 5.0,preferably from about 0.5 to about 3.0, and most preferably from about1.0 to 2.50 moles per kilogram of polymer.

The present foam structure has a cross-linked density of from 5 to 90percent and more preferably from 30 to 70 percent percent according toASTM D-2765-84, method A.

The present foam structure has density of less than 500, more preferablyless than 250 and most preferably from about 10 to about 150 kilogramsper cubic meter. The foam has an average cell size of from about 0.05 toabout 5.0, more preferably from about 0.1 to about 2.0, and mostpreferably 0.2 to about 1.0 millimeters according to ASTM D3576.

The present foam structure may be closed cell or open cell. Preferably,the present foam is greater than 90 percent closed-cell according toASTM D2856-A.

Various additives may be incorporated in the present foam structure suchas inorganic fillers, stability control agents, nucleating agents,pigments, antioxidants, acid scavengers, ultraviolet absorbers, flameretardants, processing aids, extrusion aids, and the like.

The following are examples of the present invention, and are not to beconstrued as limiting. Unless otherwise indicated, all percentages,parts, or proportions are by weight.

EXAMPLES Example 1

Cross-linked ethylenic polymer foam structures of the present inventionare made according to the process set forth below. A substantiallylinear ethylenic polymer prepared with a constrained geometry catalyst(CGCT resin) is cross-linked with dicumyl peroxide (DCP), and expandedwith azodicarbonamide to form the foam structures.

The CGCT resin, DCP, and azodicarbonamide were processed in a ModelSystem 90 Torque Rheometer mixing bowl (Haake Buchler Instruments,Inc.). The CGCT resin was an ethylene/octene copolymer 76/24 (measuredby infrared technique of ASTM 2238, method B) having a density of 0.873g/cc, melt index (I₂) of 5.2 dg/min, and I₁₀ /I₂ of 10.4. Forty grams ofthe granular resin is mixed, under nitrogen blanket, with 6 grams ofazodicarbonamide in the Haake mixer maintained at 140° C. and 20 rpmmotor speed for 5 minutes. Subsequently, predetermined amounts of DCPwere added and mixed for an additional 5 minutes. The foamable mixturewas pressed into sheet of approximately 0.080 inch (2 millimeters (mm))in thickness on a hot press at 150° C. The sheet was cut toapproximately 1/2 inch×1/2 inch (1.3 centimeters×1.3 centimeters) sizespecimens. The foamable specimens were then expanded in a hot air ovenmaintained at 230° C. for various lengths of time to determine theoptimum expansion time.

As shown in Table 1, the resin provided reasonably low density foamshaving small cell size at a relatively wide range of DCP level and ovenexposure time. Foams having densities as low as 4.8 pounds per cubicfoot (pcf) (77 kg/m³) was achieved at 0.8 DCP level. The foam was veryflexible, tough and resilient.

                  TABLE 1                                                         ______________________________________                                        Test  DCP     Expansion   Foam   Foam     Cell                                No.   Level.sup.1                                                                           Time.sup.2  Density.sup.3                                                                        Density.sup.4                                                                          Size.sup.5                          ______________________________________                                        1.1   0.5     180         24.6   423      ND                                  1.2   0.5     220         23.1   370      ND                                  1.3   0.7     180         18.5   296      ND                                  1.4   0.7     190         5.7     90      ND                                  1.5   0.7     200         14.5   232      ND                                  1.6   0.8     190         5.3     85      0.35                                1.7   0.8     230         4.8     77      0.30                                1.8   0.9     150         7.4    119      ND                                  1.9   0.9     180         7.8    125      ND                                   1.10 1.0     180         8.00   128      ND                                   1.11 1.0     190         37.4   599      ND                                  ______________________________________                                         .sup.1 Parts of dicumyl peroxide mixed in per one hundred parts of polyme     .sup.2 Time specimen allowed to expand in the oven in seconds                 .sup.3 Density of foam body in pounds per cubic foot                          .sup.4 Density of foam body in kilograms per cubic meter                      .sup.5 Cell size in millimeters determined per ASTM D3576, ND = not           determined                                                               

Example 2

Cross-linked ethylenic polymer foam structures of the present inventionwere made substantially according to the process of Example 1 except adifferent CGCT resin was used. The CGCT resin had a density of 0,888g/cc, a melt index of 3.0, and a I₁₀ /I₂ of 7.9.

As shown in Table 2, good quality foam structures of relatively lowdensity were obtained. The minimum density obtained was 6.1 pcf (98kg/m³) in Test 2.9, which was somewhat higher than the minimum obtainedin Example 1 (Test 1.7). The cell size of the foam structure in Test 2.9was 0.39 mm. The foam structures of Example 1, which utilized a higherI₁₀ /I₂ resin than those of Example 2, were more desirable than those ofExample 2 because of their lower density. The higher I₁₀ /I₂ resinprovides a higher melt elasticity, which is apparently beneficial forfoam expansion.

                  TABLE 2                                                         ______________________________________                                        Test    DCP     Expansion   Foam   Foam                                       No.     Level.sup.1                                                                           Time.sup.2  Density.sup.3                                                                        Density.sup.4                              ______________________________________                                        2.1     0.5     180         23.4   375                                        2.2     0.5     220         32.6   522                                        2.3     0.7     180         15.2   243                                        2.4     0.7     220         20.7   331                                        2.5     0.8     190         12.9   207                                        2.6     0.8     230         8.4    135                                        2.7     0.9     200         9.5    152                                        2.8     0.9     215         8.6    138                                        2.9     0.9     220         6.1     98                                         2.10   1.0     180         35.2   564                                         2.11   1.0     210         8.8    140                                         2.12   1.0     225         41.7   668                                        ______________________________________                                         .sup.1 Parts of dicumyl peroxide mixed in per one hundred parts of polyme     .sup.2 Time specimen allowed to expand in the oven in seconds                 .sup.3 Density of foam body in pounds per cubic foot                          .sup.4 Density of foam body in kilograms per cubic meter                 

Example 3

Cross-linked ethylenic polymer foam structures of the present inventionwere made substantially in accordance with Example 1 except a resinblend was employed. The resins employed were blends of a CGCT LLDPEresin and a conventional LDPE resin. The resin blend was a 50/50 blendby weight of the CG3 resin having a density of 0.92, a melt index of1.1, an I₁₀ /I₂ of 7.2 and a conventional LDPE resin having 0.916 g/ccdensity and 5.5 melt index.

A good quality foam of 4.1 pcf (66 kg/m³) and 0.41 mm cell size wasachieved at a DCP level of 0.4 pph. The results demonstrated that a lowmelt index CGCT resin could be used with an LDPE resin to make a lowdensity foam.

Example 4

A cross-linked ethylenic polymer foam structure of the present inventionwas made substantially in accordance with Example 1 with the same CGCTresin. The DCP level and expansion time were 0.8 pph and 230 seconds,respectively.

The resulting foam structure had a reasonably low density of 4.2 pcf(67.2 kgm) and a reasonably small cell size of 0.8 mm.

The foam structure exhibited desirable strength and toughnesscharacteristics. The structure had a compressive strength @ 25% (ASTMD-3575B) of 2.8 psi (pounds per square inch), a compressive set at 50%deflection (ASTM D-3575A) of 18%, a tensile strength (t.s.) (ASTMD-3575E) of 92 psi, and a tensile elongation (t.e.) (ASTM D-3575E) of394%. Toughness may be calculated according to (t.s.)×(t.e.)/(2)(100) toyield a desirably high value of 362 psi.

While embodiments of the foam structure and the process for making it ofthe present invention have been shown with regard to specific details,it will be appreciated that depending upon the manufacturing process andthe manufacturer's desires, the present invention may be modified byvarious changes while still being fairly within the scope of the novelteachings and principles herein set forth.

What is claimed is:
 1. A cross-linked ethylene polymer foam structuremade according to a process, comprising:a) forming a foamable meltpolymer material by blending and heating a decomposable chemical blowingagent and an ethylene polymer material comprising a substantially linearethylene polymer having: i) a melt flow ratio, I₁₀ /I₂, ≧5.63; ii) amolecular weight distribution, M_(w) /M_(n), defined by the equation:M_(w) /M_(n) ≧(I₁₀ /I₂)-4.63; and iii) a critical shear rate at onset ofsurface melt fracture of at least 50 percent greater than the criticalshear rate at the onset of surface melt fracture of a linear ethylenepolymer having about the same I₂ and M_(w) /M_(n) ; b) inducingcross-linking in the foamable melt polymer material; and c) expandingthe foamable melt polymer material by exposing it to an elevatedtemperature to form the foam structure.
 2. The foam structure of claim1, wherein the M_(w) /M_(n) is from about 1.5 to about 2.5.
 3. The foamstructure of claim 1, wherein the substantially linear ethylene polymerhaving from about 0.01 to about 3 long chain branches/1000 carbons alongthe polymer backbone.
 4. The foam structure of claim 3, having about0.0to about 1 long chain branches/1000 carbons along the polymerbackbone.
 5. The foam structure of claim 3, having about 0.05 to about 1long chain branches/1000 carbons along the polymer backbone.
 6. The foamstructure of claim 1, wherein the substantially linear ethylene polymeris an ethylene/alpha-olefinic copolymer.
 7. The foam structure of claim6, wherein the substantially linear ethylene polymer is a copolymer ofethylene monomer and a comonomer selected from the group consisting of1-octene, 1-butene, 1-hexene, and 4-methyl-1-pentene.
 8. The foamstructure of claim 1, wherein the substantially linear ethylene polymeris an ethylene homopolymer.
 9. The foam structure of claim 1, whereinthe foam composition is 80 percent or more closed-cell.
 10. The foamstructure of claim 1, wherein the foam composition has a density of lessthan 250 kilograms per cubic meter.
 11. The foam structure of claim 1,wherein the ethylene polymer material comprises greater than 70 percentby weight of ethylene monomeric units.
 12. The foam structure of claim1, wherein the foam composition is an extruded foam composition.
 13. Thefoam structure of claim 1, wherein the I₁₀ /I₂ is about 8 or more. 14.The foam structure of claim 1, wherein the I₁₀ /I₂ is about 9 or more.15. The foam structure of claim 1, wherein the rheological processingindex is less than or equal to 70 percent of that of a linear ethylenepolymer which lacks long-chain branching at about the same I₂ and M_(w)/M_(n).
 16. The Foam structure of claim 1, wherein the ethylene polymermaterial further comprises a low density polyethylene.
 17. The foamstructure of claim 1, wherein the ethylene polymer material furthercomprises a linear low density polyethylene.
 18. The foam structure ofclaim 1, wherein the ethylene polymer material further comprises anethylene/acrylic acid copolymer.
 19. The foam structure of claim 1,wherein the foam structure is a foam bead.